In conventional dry cleaning processes soiled fabrics and the like are washed with a substantially anhydrous dry cleaning solvent such as trichloroethylene, perchloroethylene or a petroleum base hydrocarbon such as Stoddard solvent or naphtha. The solvent effects the release of some soils from the fabric and the dissolution of solvent soluble soils, the latter group consisting primarily of fats, oils, waxes, hydrocarbons and fatty acids.
The other principal type of soluble soil which must be removed from fabrics is that which is water soluble. These soils derive chiefly from perspiration and food and beverage spillage and consist primarily of salts, carbohydrates and proteins. Small amounts of water, on the order of 0.02 to 0.12%, are frequently used in combination with the organic solvent in current dry cleaning operations. This is usually as a preparatory treatment for the removal of water soluble stains from the fabric whereby only the surface of the fabric is momentarily wetted by any water. The idea behind this minute addition of water is that the water will wet and swell water soluble stains, thereby loosening their bond with the fabric so that they can be detached by the mechanical action of the washer. In practice, such operations perform very poorly with regard to removal of water soluble soils. Also, the use of soaps in such operations to aid in removal of water soluble soils and soils which, while not water soluble are nevertheless hydrophilic, is quite inefficient due to the low water content in the solvent.
Another approach to the addition of water to organic cleaning solvents is the system wherein a detergent is used which forms a stable colloidal solution, in the solvent, of aggregates of molecules called micelles. These micelles possess a hydrophilic interior and are capable of dissolving water which, in turn, is capable of dissolving water soluble soil. Thus, water is completely solubilized by a dry-cleaning detergent but the water content is still low and consequently such systems also have very limited removal power for water soluble soils. This so-called "charged system" has been largely abandoned in modern cleaning practices and water is now added batchwise as an emulsion with the organic solvent, as a spray or in the form of steam. The amount of water added varies from about 0.5 to about 7%, calculated on the weight of fabric, depending upon the particular cleaning application.
Thus, in fabric cleaning operations, the solvent charge, inter alia, serves the dual role of removing non-aqueous soluble soils as well as water soluble soils. While the preponderance of non-aqueous content of the solvent can readily handle the non-aqueous soluble soils, the small amounts of water addition to which most current practices are limited severely curtails the effective simultaneous removal of water soluble soils. This deficiency causes severe problems in the fabric cleaning industry. Following cleaning with a solvent of inadequate water soluble soil removing capability, an article must either be returned as is or subjected to expensive "wet-cleaning" whereby water soluble soil which has become imbedded in the fibers is wetted by an aqueous detergent solution and gently worked by hand brushing. Obviously, neither course is desirable.
Thus, small amounts of water are often used in fabric cleaning but the water is always either completely dissolved in micellar form or in such small quantity that only the surface of the fabrics can be momentarily wetted. Consequently, the result in conventional practice has been that the formulation of cleaning solvents has compromised the thoroughness with which fabrics can be cleaned because such solvents contain too little water.
It has been pointed out in the literature that two-phase or emulsion cleaning, utilizing an emulsion of an organic cleaning composition with a high water content and a suitable surfactant, provides remarkably good cleaning characteristics. This two-phase approach, with regard to the critical parameters of soil removal and redeposition, achieves markedly superior results to those obtained using pure organic solvent (drycleaning) or cleaning in aqueous media (laundering). Further testing has demonstrated the direct relationship between increasing the water content in such a two-phase cleaning solvent emulsion and the improvement in the overall cleaning performance.
While there are certain ancillary problems in using a high water content emulsion cleaning solvent (eg., difficulty in filtration to remove insolubles), the primary reason why such emulsions are not routinely utilized is that the post-use recovery of the expensive organic fraction is uneconomical.
The principal method of purification of cleaning solvents in the United States is by distillation. Following separation of the solvent from the cleaned fabrics it may first be filtered by various means to remove insoluble material which has been picked up in the cleaning process. Eventually, however, the solvent will be cleansed by distillation, a very costly operation. The distillation is necessary to remove solvent soluble contaminants from the cleaning solvent. Also removed are insoluble materials which have not previously been separated from the solvent by filtration. The distillation carried on in conventional cleaning operations effects delivery of the liquid cleaning solvent, including the water fraction thereof, to a separating device wherein the water and organic fractions are separated prior to succeeding reuse of the expensive organic fraction. The water fraction is discarded or reused depending upon the nature of the operation, the volume involved etc.
It is clear that a large increase in the water content of the solvent to be distilled will dramatically increase the cost/benefit ratio of the distillation operation since the distilled water has little value relative to the organic fraction and particularly because water requires about ten times as great a thermal input to distill than does an organic solvent such as perchloroethylene. Thus, the fabric cleaning industry is faced with the desirability of using high water content solvents from the standpoint of optimizing cleaning capability while simultaneously being unable to do so without such large expenditures for energy that the cleaning process itself may be rendered uneconomical. Clearly, the problem of efficient solvent separation and recovery is of critical importance to any practical application of two-phase emulsion cleaning.
Another major problem encountered in the fabric cleaning industry, particularly in the cleaning of heavily soiled industrial work, is the removal of large quantities of insoluble soil and contaminants from the solvent prior to reuse. Fabrics to be cleaned such as work overalls, shop towels, floor mats, mops and the like contain up to ten times the amount of insoluble soil found in lighter work such as suits, dresses and household items. Such high concentrations of insoluble soil exceed conventional filter capacity and lead to filter clogging, solvent backup and excessive equipment downtime. Of the attempts to deal with the problem of high solvent contamination by insoluble matter, distillation in on-site stills has been the most successful. However, in removal of such large quantities of insoluble matter the efficiency of a still decreases rapidly due to encrustations which form on heat exchange surfaces. As with the filtration problems with heavy insoluble content solvent, such solvents also necessitate expensive shutdowns of distillation equipment. Thus, it would be desirable to deliver solvent to the still with as small a content of insolubles as possible.
It is an object of this invention to introduce a process of fabric cleaning which renders the use of cleaning solvents with functionally optimal water contents economically practicable.
Another object of the invention is to largely obviate the need for solvent filtration steps in fabric cleaning processes.
A further object of the invention is to provide a fabric cleaning process wherein an organic drycleaning composition is delivered for distillation cleansing with a low content of insoluble impurities.